(1) Field of the Invention
This invention is directed to the field of spherical plug valves. The invention is particularly directed to improvements to the structure and method of manufacturing the plug and body seats of spherical plug valves.
(2) Description of the Related Art
Spherical plug valves are well known in the prior art. This type of valve is used in fluid piping to start and stop the flow of fluid through piping by opening and closing the valve. The diameter of piping used with this valve ranges from a fraction of an inch to several feet in diameter; however, the subject invention is primarily directed to a spherical plug valve for fluid applications and for piping of a foot or more in diameter.
Typically, a spherical plug valve is comprised of a plug which rotates within a valve body to open and close the valve. The valve body generally has two orifices, although there may be more if the specific application requires. One of the orifices is the inlet port and the other is the exit port. Usually, these ports have a common axis or axes oriented 90.degree. apart depending upon the specific application. The inlet port typically has an annular body seat, the function of which will be described later. The exit port may also have a body seat.
Generally, the valve plug includes a valve stem and a stub shaft, each having bushings, bearings and seals. The valve stem and coaxial stub shaft extend from opposite sides of the valve plug and through the bushings, bearings, and seals which mount the plug to the valve body. The bushings, bearings, and seals permit the plug to freely rotate within the valve body and seal the valve stem and stub shaft from leakage. As is well-known in the art, the valve stem is connected to a drive mechanism to rotate the stem through an arc and thereby rotate the plug within the body between the open and closed positions. At least one of the bearings is generally a thrust bearing to compensate for any axial forces which may be imparted to the valve plug by valve orientation, hydrostatic forces or an actuator force component of the drive mechanism.
The spherical valve plug is a species of plug valve which does not necessarily have a spherical plug. In fact, in the larger embodiments, the plug is preferably not spherical because a spherically-shaped plug would not be optimal in terms of conservation of material. Instead, the plug is more cylindrically shaped with the axis of the cylinder being perpendicular to the axis of plug rotation. The plug usually has a hollow bore extending along the cylinder axis, the bore having approximately the same interior diameter as the piping. The bore is configured to extend between the inlet port and the exit port of the valve body when the valve plug is in the open position. The plug also includes an annular plug seat which mates with the body seat to prevent or inhibit fluid flow through the valve when the plug is in the closed position. Thus, fluid flows through the plug bore between the inlet and exit ports of the valve body when the valve plug is in the open position and fluid is prevented from flowing through the valve body by the plug seat sealing against the body seat when the valve is in the closed position.
In order to effectively seal the plug seat and body seat, the valve plug is typically manufactured such that the center of rotation of the plug is not coincident with the center of the valve body. Instead, the center of rotation of the plug is slightly eccentric so that the relative motion between the plug seat and body seat is not entirely tangential as the plug enters the closed position. Thus, the plug and body seats will clamp shut and tightly seal against one another even with larger manufacturing tolerances and after the seats have worn.
Originally, the prior art seats were constructed of metals having significant hardness, but this required relatively tight tolerances on the seating surfaces or seal leakage would occur. In order to solve this problem, prior art seats for either the valve body or valve plug were formed incorporating a thin, resilient, and flexible rim around the seat. As the valve plug moved to the closed position the resilient rim would flex a little due to differential pressure acting on the back side, or high pressure side, of the rim. This would cause the seat rim to make contact with the other seat and close the valve. However, if a hard foreign object or piece of debris was trapped between the seal surfaces during valve closure, the flexible rim of the seat could become damaged or deformed due to its thin construction. Once deformed, the seat rim would not subsequently produce an effective seal. Thus, the resilient seat rim was typically made to be replaceable and was fastened in place by bolts, screw threads or shrink fit in place.
In order to solve the durability problem with resilient metallic seats, the inventor previously developed a resilient valve plug seat which consists of a rigid metallic seat ring connected to the valve plug by an elastomeric cylinder. The elastomeric cylinder permitted the seat ring to move relative to the plug and thereby compensated for manufacturing tolerances and wear. Thus, the improved plug seat with the elastomeric cylinder provided an adequate seal and was sufficiently rigid to prevent significant damage or deformation to the seat when a foreign object became trapped between the mating surfaces of the plug seat and body seat during valve closure. This elastomeric cylinder and plug seat has been the state-of-the-art from 1991 until the present. However, in order to refurbish this valve plug and return the plug seat surface to within originally manufactured tolerances, the valve must be taken out of service. The entire plug is then removed and replaced with a replacement plug having a plug seat with the original specifications. The worn plug can then be returned to the factory where the elastomeric cylinder and plug seat can be removed and a new plug seat and elastomeric cylinder can be installed.
The elastomeric cylinder is manufactured by inserting liquid silicone rubber between the plug seat and plug and permitting it to cure thereby forming the elastomeric cylinder from the silicone rubber. The periphery of the plug seat and the valve plug surface surrounding the seat and adjacent the elastomeric cylinder have circumferential serrations to achieve a mechanical bond of the elastomeric cylinder to the plug seat and plug surface. Air bubbles are often present in the silicone rubber after curing and can provide an initiation site for subsequent elastomeric cylinder failure. Thus, on occasion an elastomeric cylinder can develop leaks due to progressive collapse of the bubbles or voids in the elastomeric cylinder.
Due to the eccentricity between the center of rotation of the plug seat sealing surface and the valve body center, the body seat sealing surface is also eccentric relative to the valve body center. In prior art designs, the body seat was manufactured separately from the body and fastened in place. Due to the eccentricity of the body seat relative to the body, the body seat was machined eccentrically. This resulted in the body seat having different width dimensions about its circumference. This caused body seat manufacturing costs to be higher than they would be otherwise without the machined eccentricity. After the body seat and body were properly machined, they were typically fastened together using an epoxy polymer. As with the plug and plug seat, serrations were provided in the cylindrical body surface surrounding the body seat and in the cylindrical periphery of the body seat to improve the mechanical connection of the epoxy to the serrated surfaces. The surfaces of the valve body and the body seat which were connected together were typically cylindrical and an O-ring was positioned between the body surface and the periphery of the seat to prevent the epoxy from escaping from between the valve body and seat. Due to the cylindrical configuration of the valve body surface and the periphery of the body seat, the O-ring occasionally became pinched or torn during body seat insertion destroying its ability to seal between the valve body and body seat surfaces. In the event of O-ring pinching or tearing, the body seat had to be removed, a new O-ring had to be installed if necessary, and the body seat had to be reinstalled. This corrective process had to be performed prior to the epoxy curing or it would harden. In that event, the epoxy had to be removed prior to beginning the assembly process over. Thus, this process could become time consuming and expensive.
The present invention overcomes the disadvantages of the prior art spherical plug valves and provides a unique spherical plug valve apparatus and assembly method which improves performance, manufacturability, and maintainability.